GABA is the most abundant inhibitory neurotransmitter in the mammalian brain. GABA controls brain excitability by exerting inhibitory functions on neuronal membranes by altering their permeability to specific ions. Binding of GABA to the GABAA-type (GABAA) receptor increases the permeability of neuronal membranes to chloride ions (Cl—). In most neurons the relative Cl— ion concentration is greater outside than the inside the membrane. Thus, selective permeability to Cl— initiated by GABA binding allows Cl— to flow down its electrochemical gradient into the cell. The majority of fast inhibitory synaptic transmission is a result of GABA binding to the GABAA receptors. GABAA receptors are ubiquitously expressed throughout the CNS with almost all neurons staining for their presence. The GABAA receptor is a hetero-pentameric protein structure of the nicotinic acetylcholine receptor superfamily. Native GABAA receptors are formed from at least 19 related subunits. The subunits are grouped into α, β, δ, ε, π, and ρ families. The most prevalent combination of GABAA receptors is a stoichiometric combination of the 2×α, 2×β, and 1×γ subunits, with the remaining subunits relegated to substituting for the γ subunit during specific development expression or in highly specific brain region localization. The adult brain predominately express the α1β2γ2 subunit combination (60%) with the α2β3γ2 and α3βnγ2 subunits comprising the majority (35%) of the remaining receptors. The relative effects of GABA are influenced by the GABAA receptor subunit expressed in a specific brain region or neuronal circuit.
The neurophysiological effects of GABA result from a conformational change that occurs when GABA binds to the GABAA receptor. The GABAA receptor and the associated ion channel complex (GRC) is a ligand-gated ion channel which recognizes many compounds that allosterically modulate the ability of GABA to bind to the GABAA receptor. The allosteric modulators have distinct sites on the GRC. These sites are separate and unique from the site that recognizes GABA. The most widely studied and characterized class of allosteric modulator of the GRC is that which interact with the benzodiazepine (BZ)-site.
Alternative sites for modulating the GRC have been described. For example, neuroactive steroids are non-hormonal steroids that bind and functionally modulate the GRC. The current role of neuroactive steroids in GABAA receptor pharmacology is supported by overwhelming evidence. Electrophysiological and biochemical techniques have confirmed the capacity of neuroactive steroids to allosterically modulate the GRC through a unique site of action. Experimentally neuroactive steroids exhibit a pharmacological profile similar, but not identical, to the benzodiazepines. Neuroactive steroids produce anxiolytic, anticonvulsant, and sedative-hypnotic properties.
Certain antibacterial fluoroquinolone antibiotics have been implicated in clinical reports as the cause of convulsions in humans (Ball P (1986) Journal of Antimicrobial Chemotherapy. 18 Suppl D 187-193; Simpson K J, Brodie M J (1985) Lancet ii: 161, 1985; Hori S, et al. (1 987) Program and Abstracts of the Twenty-Seventh Interscience Conference on Antimicrobial Agents and Chemotherapy, New York 1987. Abstract 30, pg 101). Experimentally, fluoroquinolones have been demonstrated to produce convulsions and death in mice. Additionally, non-steroidal anti-inflammatory drugs (NSAIDs) and their by-products have been reported to clinically and experimentally potentiate the convulsive effects of the fluoroquinolones. Concerns about the convulsant side-effects of fluoroquinolone antibacterial agents have led to an interest in the interaction of fluoroquinolones with the GABAA receptor. Convincing evidence has accumulated that suggests that they interact with the GRC to inhibit GABA action. Fluoroquinolones antagonize [3H]muscimol and [3H]GABA binding to the GRC with high micromolar potency. Electrophysiological studies have demonstrated that fluoroquinolones alone weakly reduce GABA-evoked currents. As well, radioligand binding assays have shown that fluoroquinolones, in combination with NSAEDs, induce a conformational change in the GABAA receptor-chloride channel complex that is indicative of a pharmacologically relevant response consistent with functional antagonism of GABA.
It is well-documented that modulation of the GRC can ameliorate anxiety, seizure activity, and insomnia. Thus, GABA and drugs that act like GABA or facilitate the effects of GABA (e.g., the therapeutically useful barbiturates and benzodiazepines (BZs) such as Valium) produce their therapeutically useful effects by interacting with specific modulatory sites on the GRC. None of the known drugs, however, are selectively potent at the α-2 subunit of the GABA receptor. Thus, they exhibit undesirable side effects of sedation, and in the case of fluoroquinolones, convulsions. There is presently a need for GRC modulators that are active without side effects.